Working in the tile industry I guess I can help you with this one. You don’t need to measure the gaps, you just have to use these small things called spacers(refer to the image). Having some space in between the tiles makes it safe( as same as the gapes in railway tracks) and it also adds to the looks. Sizes available 2mm, 3mm, 4mm,5mm,6mm,8mm, 10mm, and 12 mm. You can know the exact size or else hit depending on what size and material( vitrified or ceramic) you are using and at what place( floor/wall).
In fact, many factors need to be considered in this issue, but we can help you more with the choice of tile grout. A good tile grout has many benefits.
If you want to get more information, welcome to contact KASTAR!